1. Field of the Invention
The present invention relates to an injection unit of an injection molding machine.
2. Description of the Related Art
A prior art injection unit of an injection molding machine comprises a base, heating cylinder, front plate secured to the base, and rear plate connected to the front plate by tie bars. A pusher plate fitted with an injection screw, which is rotatable and axially immovable, is disposed between the front and rear plates. A drive mechanism for advancing and retracting the pusher plate is disposed between the pusher plate and the rear plate. With this configuration, the pusher plate advances and retreats, guided by the tie bars, to move the injection screw forward and backward.
Normally, in the injection unit of an injection molding machine constructed in this manner, the posture of the pusher plate should be adjusted to align the axes of the heating cylinder attached to the front plate with the injection screw attached to the pusher plate. Techniques for adjusting the posture of the pusher plate of this type are known from the following patent documents.
Japanese Patent Application Laid-Open No. 10-217297 discloses a technique in which a clearance is formed between a connecting bolt and a pressure plate to prevent contact between the two so that the accuracy of pressure detection by a load cell cannot be damaged.
In the technique described above, however, the position of the pusher plate cannot be adjusted despite the presence of the clearance between the connecting bolt and the pressure plate.
Japanese Patent Application Laid-Open No. 2002-166443 discloses a technique for an electric injection mechanism, in which alignment members are disposed on the opposite sides of a lower part of a screw drive body to align an injection heating cylinder with the screw drive body. The alignment members are each provided with height adjusting means and serve to slidably support the screw drive body in conjunction with an installation member. Further, Japanese Patent Application Laid-Open No. 2001-269974 discloses a technique for an injection unit of an injection molding machine, in which a linear guide with height adjustability is provided at a lower part of a moving plate to prevent downward misalignment of the moving plate.
In the techniques described above, the pusher plate is supported by the installation member or the moving plate. Even though only the pusher plate is adjusted, misalignment between the axes of the heating cylinder and the screw may occur depending on variation in the accuracy of each component, or an unbalanced load applied to a ball screw portion may occur. In contrast with this, although the position of the pusher plate can be adjusted so that the axis of the screw is aligned with that of the heating cylinder attached to the front plate, the tie bars may be deflected or a ball screw nut connected to the pusher plate and a ball screw shaft connected to the rear plate may be misaligned. Consequently, an unbalanced load may be applied to the ball screw, thereby reducing its service life.
Japanese Patent Application Laid-Open No. 2000-218663 discloses a technique in which a screw holding member and a pusher plate are connected by means of bolts to each other through a load cell, the screw holding member is supported by tie rods, and the pusher plate is supported by the tie rods and guide rods.
In the technique described above, however, both the screw holding member and the pusher plate are supported by the tie rods. Therefore, both the loads of the screw holding member and the pusher plate are inevitably applied to the tie rods, so that the tie rods may be deflected. As a result, there may be caused misalignment between the heating cylinder and the screw, increase in the sliding resistance of the pusher plate, or reduction in the service life of a drive mechanism, such as a ball screw for driving the pusher plate, due to an unbalanced load.